Network Topology of Symbolic and Nonsymbolic Number Processing: A 7T fMRI Study

There is a longstanding debate regarding the extent to which symbolic (e.g. Arabic digits) and nonsymbolic (e.g. dot arrays) numbers engage shared versus distinct neural mechanisms. Previous functional magnetic resonance imaging (fMRI) studies have almost exclusively assessed regional activation with only a few evaluating functional connectivity via psychophysiological interaction analysis. Graph theory provides a methodological framework for describing the network architecture of the brain, with potential to shed new light on the symbolic/nonsymbolic number debate. Here, we employ graph theoretical measures to assess the networks engaged during symbolic and nonsymbolic number processing. We conducted an event-related 7T fMRI study with healthy subjects (n=40, 19.5 ± 0.9 years). Participants performed a number comparison task in which they decided whether Arabic digits or dot arrays were more/less than five. To assess task-related functional connectivity, we performed beta series correlations. Average beta series were extracted from a whole-brain, 246 region atlas and separated based on condition. Connectivity matrices were constructed for each subject/condition and regions were partitioned into functional modules according to consensus clustering. For both conditions, six modules were delineated including a fronto-parietal, default mode, visual, sensorimotor, temporal and subcortical network. 26 of 246 regions were assigned to different modules between conditions. These differences were primarily characterized by a more distributed fronto-parietal network in the nonsymbolic condition, with 19 additional regions from parietal, frontal, and temporal areas being included in this network compared to the symbolic condition. Our findings suggest organizational differences in whole-brain functional network architecture between nonsymbolic and symbolic processing.